A conventional sawing machine, such as those of the type disclosed in U.S. Pat. No. 6,595,095, U.S. Pat. No. 6,474,206, and China Patent No. 201061835, mainly includes a machine base unit for supporting a workpiece for cutting, a biased swinging arm pivotally mounted at one side of the machine base unit and pivotal about a pivot axis and swingable about a swinging axis, and a saw blade rotatably mounted to the swinging arm and rotatable about a rotation axis and adapted for cutting a workpiece. By way of the pivotability of the swing arm and a rotating function of the machine base unit, the saw blade can cut the workpiece at a tilted cutting angle as desired, or the saw blade can be adjusted relative to the workpiece to any desired cutting angle.
FIG. 1 illustrates a miter saw constructed according to U.S. Pat. No. 6,595,095. According to this design, the miter saw includes a fixed base 10, a bench 11 rotatably mounted on the base 10, an extension holder member 12 movable with the bench 11 and extending out of and away from the base 10, a cutter unit (not shown) mounted on the bench 11, and a positioning device set 13. The positioning device set 13 includes a positioning member 131 inserted through the extension holder member 12, a lever 132 pivotally connected to the extension holder member 12, and a link 133 pivotally coupled between the positioning member 131 and the lever 132.
Thus, when the lever 132 is rotated downwards, the link 133 causes the positioning member 131 to move away from the base 10, allowing free movement of the extension holder member 12 in order to adjust the angle of the bench 11, and hence the workpiece, relative to the cutter unit. When the lever 132 is rotated upwards, the link 133 causes the positioning member 131 to move into engagement with the base 10, thereby locking the bench 11 from rotation relative to the base 10.
The aforesaid positioning device set 13 can be operated to lock or unlock the bench 11. However, because the tightening pressure of the positioning member 131 is determined subject to the amount of extension of the lever 132 and the link 133 and because two pivot points A and B are provided between the positioning member 131, the lever 132, and the link 133, the reaction force produced upon engagement between the positioning member 131 and the base 10 will be concentrated at the pivoted points A and B. If the extension amount of the link 133 is too small, the tightening pressure will be insufficient to lock the rotation of the bench 11 with respect to the base 10. If the extension amount of the link 133 is too large, a large effort will be needed to rotate the lever 132 and therefore, structural damage may occur.